1. Field of the Invention
This invention relates to a flow meter such as a device for detecting the rate of a flow of fluid. This invention relates to, for example, an air flow meter provided in an air induction passage of and internal combustion engine.
2. Description of the Prior Art
Recently, some engines have been subjected to microcomputer-based general control to improve their performances. In the case of automotive engines, general engine control includes control of the A/F (air-to-fuel) ratio of an air-fuel mixture supplied to the engines and control of the rate of the fuel injection into the engines.
Usually, the A/F ratio control and the fuel injection rate control require information of the rate of the flow of air drawn into the engines. In typical cases, air flow meters disposed in air induction passages of the engines detect the rate of the air flow into the engines. Output signals of such air flow meters are used in the A/F ratio control and the fuel injection rate control as an indication of the rate of the air flow into the engines.
Some of air flow meters use a flow sensor of the hot-wire type. Usually, hot-wire flow sensors are advantageous in cost as well as dynamic range.
A prior-art air flow meter of the hot-wire type has a cylindrical casing formed with a main air passage and a sub or branch air passage which by-passes the main air passage. The cross sectional area of the main passage and the cross sectional area of the branch passage are in a predetermined relation so that the rate of the air flow through the branch passage has a given relation with the rate of the air flow through the main passage. Accordingly, only the rate of the air flow through the branch passage is detected, and the sum of the rates of the air flows through the main passage and the branch passage is estimated from the detected air flow rate related to the branch passage.
Regarding the prior-art air flow meter, a hot-wire resistor for measuring an air flow rate is disposed in the branch passage. A resistor for temperature compensation is also disposed in the branch passage. The two resistors extend parallel to each other but occupy different places as viewed in the direction of the air flow. The two resistors are electrically connected to a control circuit via leads.
In the prior-art air flow meter, the hot-wire resistor is exposed to only the air flow in the branch passage. Therefore, the prior-art air flow meter is advantageous in preventing the hot-wire resistor from being contaminated by dust in air. In the case of automotive use, the prior-art air flow meter follows an air cleaner element in the direction of the air flow. The design including the branch passage is effective in reducing a disturbance in the velocity distribution of the air flow which is caused by the air cleaner element.
In the prior-art air flow meter, it is difficult to completely remove such a disturbance from the velocity distribution of the air flow. In addition, it is difficult to remove pulsations from the air flow which are caused by operation of a related automotive engine.
In automotive engines, a backfire tends to occur when an ignition timing is excessively earlier than the normal timing. During the occurrence of a backfire, shock wave caused by a high pressure and a high temperature in engine cylinders travels back toward an upstream side of a related engine. The hot-wire resistor in the prior-art air flow meter tends to be damaged or deformed by backfire shock wave. In addition, dust on the hot-wire resistor is burned by a backfire and the resultant material is closely fixed to the hot-wire resistor so that the hot-wire resistor is deteriorated.
Japanese published unexamined patent application 56-108910 discloses an advanced air flow meter provided with an air flow buffer extending upstream of a hot-wire resistor but downstream of an air cleaner element. The air flow buffer reduces disturbances and pulsations in an air flow which meets the hot-wire resistor. The air flow buffer includes a stainless steel mesh or a ceramic honeycomb. Generally, the air flow buffer effectively damps backfire shock wave which travels between the engine and the air cleaner element. Accordingly, the air flow buffer protects the hot-wire resistor from the backfire shock wave.
In a range around a certain point of the air flow rate, the value represented by the output signal of the air flow meter of Japanese application 56-108910 tends to vary discontinuously as a function of the air flow rate.